The Prokaryotic Species Problem is Even Worse

Ford Doolittle & Olga Zhaxybayeva

Although some organisms form genotypic and phenotypic clusters tight enough that we
might want to call them “species”, there is no single process causing such clusters
to form, and for some datasets taken to delimit species, a purely random birth-death
explanation cannot be rejected. Although polyphyletic groupings are not acceptable
as taxonomic species, and some form of “metapopulation lineage” requirement might
be universally applicable, lineage concepts provide no principled way to distinguish
“species” from “varieties” on the one hand and “genera” on the other, and hold up
poorly in the face of promiscuous inter-taxon (“lateral”) gene transfer. None of these
problems is unique to prokaryotes but microbiologists have often written as if they
were, and indeed there may be differences in degree, prokaryotes to eukaryotes. Sensible
microbiologists now preoccupy themselves with studying speciation, while finessing
any definition of ‘species’.